Hydrocarbon saturation determination using acoustic velocities obtained through casing

Abstract

Compressional and shear velocities of earth formations are measured through casing. The determined compressional and shear velocities are used in a two component mixing model to provides improved quantitative values for the solid, the dry frame, and the pore compressibility. These are used in determination of hydrocarbon saturation.

1. A method of evaluating an earth formation having at least two components in a matrix of the formation, the method comprising:
 (a) measuring a compressional velocity and a shear velocity of the formation through a casing in a borehole in the earth formation; 
 (b) measuring a porosity of the earth formation through the casing; 
 (c) measuring a fraction of one of the at least two components of the earth formation through the casing; and 
 (d) using a two-component mixing model, the measured compressional velocity, the measured shear velocity, the measured porosity and the measured fraction of the one component of the earth formation to estimate a property of a fluid in the formation;
 wherein each component of the two-component model satisfies a relation of the form: 
 
 
     
       
         
           
             
               
                 K 
                 sat 
               
               
                 
                   K 
                   o 
                 
                 - 
                 
                   K 
                   sat 
                 
               
             
             = 
             
               
                 K 
                 
                   
                     K 
                     m 
                   
                   - 
                   K 
                 
               
               + 
               
                 
                   K 
                   f 
                 
                 
                   ϕ 
                   ⁡ 
                   
                     ( 
                     
                       
                         K 
                         o 
                       
                       - 
                       
                         K 
                         f 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
       where φ is a porosity, K is a frame modulus, K m  is a grain modulus, and K f  is a fluid modulus. 
     
   
   
     2. The method of  claim 1  wherein the one of the at least two components comprises a shale and measuring the fraction further comprises making a measurement of natural gamma radiation in the borehole. 
   
   
     3. The method of  claim 1  wherein estimating the property of the fluid further comprises estimating at least one of (i) a compliance of a fluid, and (ii) a saturation of a fluid in the earth formation. 
   
   
     4. The method of  claim 1  wherein using the model further comprises a distribution of a total porosity between the at least two components. 
   
   
     5. A method of evaluating an earth formation having at least two components in a matrix of the formation, the method comprising:
 (a) measuring a compressional velocity and a shear velocity of the formation through a casing in a borehole in the earth formation; 
 (b) measuring a porosity of the earth formation through the casing; 
 (c) measuring a fraction of one of the at least two components of the earth formation through the casing; and 
 (d) using a two-component mixing model, the measured compressional velocity, the measured shear velocity, the measured porosity and the measured fraction of the one component of the earth formation to estimate a property of a fluid in the formation; 
 further comprising defining the model by: 
 (i) obtaining measurements of a compressional velocity and a shear velocity for a training set; and 
 (ii) selecting a model parameter that improves a match between a predicted velocity from the model and at least one of (A) a measured compressional velocity for the training set, and (B) a measured shear velocity for the training set. 
 
   
   
     6. The method of  claim 5  wherein selecting the model parameter further comprises selecting a sample from the training set that has a value close to 100% of one of the at least two components. 
   
   
     7. The method of  claim 1  further comprising identifying an interval in the borehole having a high saturation of a fluid in the formation. 
   
   
     8. The method of  claim 1  wherein the property of the fluid further comprises a hydrocarbon saturation, the method further comprising perforating an interval of the borehole based on the estimate hydrocarbon saturation. 
   
   
     9. The method of  claim 1  wherein the property of the fluid comprises a gas saturation, the method further comprising altering a fluid pressure at another borehole. 
   
   
     10. A system for evaluating an earth formation having at least two components in a matrix of the formation, the system comprising:
 (a) a logging tool conveyed in a cased borehole in the earth formation, the logging tool configured to make a measurement of a shear velocity of the formation, a compressional velocity of the formation, a porosity of the earth formation, and a fraction of at least one of the two components of the matrix; and 
 (b) a processor configured to use a two-component mixing model, the measured compressional velocity, the measured shear velocity, the measured porosity and the measured fraction of the at least one component of the earth formation to estimate a property of a fluid in the formation; 
 wherein the processor is further configured to define each component of the two-component model using a relation of the form: 
 
     
       
         
           
             
               
                 K 
                 sat 
               
               
                 
                   K 
                   o 
                 
                 - 
                 
                   K 
                   sat 
                 
               
             
             = 
             
               
                 K 
                 
                   
                     K 
                     m 
                   
                   - 
                   K 
                 
               
               + 
               
                 
                   K 
                   f 
                 
                 
                   ϕ 
                   ⁡ 
                   
                     ( 
                     
                       
                         K 
                         o 
                       
                       - 
                       
                         K 
                         f 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     where φ is a porosity, K is a frame modulus, K m  is a grain modulus, and K f  is a fluid modulus. 
   
   
     11. The system of  claim 10  wherein the one of the at least two components comprises a shale and wherein the logging tool is configured to measure the fraction by further being configured to make a measurement of natural gamma radiation in the borehole. 
   
   
     12. The system of  claim 10  wherein the processor is further configured to estimate at least one of (i) a compliance of a fluid, and (ii) a saturation of a fluid in the earth formation. 
   
   
     13. The system of  claim 10  wherein the processor is further configured to use the model by defining a distribution of porosity between the at least two components. 
   
   
     14. A system for evaluating an earth formation having at least two components in a matrix of the formation, the system comprising:
 (a) a logging tool conveyed in a cased borehole in the earth formation, the logging tool configured to make a measurement of a shear velocity of the formation, a compressional velocity of the formation, a porosity of the earth formation, and a fraction of at least one of the two components of the matrix; and 
 (b) a processor configured to use a two-component mixing model, the measured compressional velocity, the measured shear velocity, the measured porosity and the measured fraction of the at least one component of the earth formation to estimate a property of a fluid in the formation; 
 wherein the processor is further configured to define the model by: 
 (i) obtaining measurements of a compressional velocity and a shear velocity for a training set; and 
 (ii) selecting a model parameter that improves a match between a predicted velocity from the model and at least one of (A) a measured compressional velocity for the training ser, and (B) a measured shear velocity for the training set. 
 
   
   
     15. The system of  claim 14  wherein the processor is further configured to select the model parameter by using a sample from the training set that has a value close to 100% of one of the at least two components.